Electrogasdynamic particle deposition systems

ABSTRACT

MEANS AND METHODS ARE DISCLOSED FOR EXTRACTING CHARGED PARTICLES FROM A HIGH VELOCITY AIR STREAM SO AS TO UNIFORMLY COAT A SUBSTRATE. THE PARTICLES ARE ENTRAINED IN A MOVING GASEOUS MEDIUM, CHARGED BY AN IONIZING ELECTRIC FIELD, DIRECTED AT A BEND ZONE INTO A NON-TURBULENT FLUID REGION, AND DRIVEN BY A REPELLING ELECTRIC FIELD TO A DEPOSITION SURFACE.

June 15, 1971 Q GQURDINE EI'AL 3,585,050

ELECTROGASDYNAMIC PARTICLE DEPOSITION SYSTEMS Filed Jan. 24, 1969 2 Sheets-Sheet 1 MEREDITH c. GOURDINE, GUY R. MORTON B Y ML G MSW fheir A TTOR/VEYS.

June 15, 1971 c, GQURDlNE ETAL 3,585,060

ELECTROGASDYNAMIC PARTICLE DEPOSITION SYSTEMS Filed Jan. 244 1969 2 Sheets-Sheet 2 INVliNlYWS TO VA P F CUUM UMP 3 MEREDITH C. GOURDINE GUY R. MORTON W. M,MM

their A 7'7'OR/VEY5 United States Patent 3,585,060 'ELECTROGASDYNAMIC PARTICLE DEPOSITION SYSTEMS Meredith C. Gourdine, West Orange, N.J., and Guy R. Morton, Bronx, N.Y., assignors to Gourdine Systems, Incorporated, Livingston, NJ.

Filed Jan. 24, 1969, Ser. No. 793,729 Int. Cl. Bb 5/02 US. Cl. 117-17 5 Claims ABSTRACT OF THE DISCLOSURE Means and methods are disclosed for extracting charged particles from a high velocity air stream so as to uni formly coat a substrate. The particles are entrained in a moving gaseous medium, charged by an ionizing electric field, directed at a bend zone into a non-turbulent fluid region, and driven by a repelling electric fieldto a deposition surface.

. This invention relates toa method of extracting and efficiently depositing charged particles from a high velocity air stream.

There are many items of manufacture today which require somewhere in their manufacture one substance to be coated on another. There are many different methods by which the coating process is carried out. The variables which determine the coating method to be used are the substrate to be coated, the material to be coated on the substrate, and the ultimate use of the coated item.

. It has been proposed by at least ourselves to employ electrogasdynamic systems to improve those coating operations. Such electrog-asdynamic coating systems are adaptable to almost all conventional application requirements. Those are: coating of surfaces with plastic or metal powders; liquid coating system; deposition of micron thick films on paper, steel sheets, etc.; and all industrial coating applications now requiring solvents and curing ovens. The use of such electrogasdynamic systems results in greater ease of application, speed of coating, and cost savings, 4

In general, an electrogasdynamic coating process for particulate or aerosol deposition is accomplished by bringing a carrier gas stream, for example air, containing entrained charged particles of the material to be used in the coating in contact with the substrate to be coated. The particles are charged by conveying the gaseous stream containing the particles past a region in which a corona discharge has been established. The particles are driven to and adhered to the substrate by means of an electric field or space charges. 1

Where the coating thickness for powders are DOS-.010 inch or greater, the turbulence at the substrate caused by the high velocity gaseous stream does not effect the uniformity of the coating. However, where the coating thicknesses are less than .005 inch the turbulence can be substantially detrimental to the coating process.

More specifically, such turbulence is .detrimental to many processes in which it is desired to have the charged particles deposited in a controlled manner on specified surfaces. For example, in the present manufacturing process for color television tubes, a tube blank is coated with a photo-sensitive polymer and dusted with phosphor to a prescribed thickness on the order of 5 mg./cm. then the polymer is sensitized by a light source and the remaining uncured polymer and excess phosphor is washed away. At the required mass loading of 5 mg./cm. for phosphor deposition, the thickness of the coating can be less than .001 inch depending on the phosphor packing fraction and density. Experience has shown that the presence of turbulence significantly effects the uniformity of a .001-

inch coating. Nonuniform deposition of phosphor on the tube faces has resulted in a rejection rate of over 30% by one manufacturer.

The present invention overcomes the aforementioned problems by providing means and methods whereby the particles to be deposited are ejected from the high velocity air stream into a region of still air, eliminating the detrimental effects of turbulence. The desired uniform coating is achieved by placing the subtrate to be coated within this region of non-turbulent air.

Basically, the invention consists of incorporating the particles to be deposited in a moving gaseous medium, exposing the particles to an ionizing electric field Where they acquire a charge, and deflecting the flow of the medium containing the particles so as to allow the particles to be directed through an aperture into a non-turbulent fluid region. The particles can be charged by the ionizing field either before or after separtion thereof from the moving medium. The separated, charged particles are driven by a repelling electric field to the deposition surface. Such repelling field may be produced by means external to the particles and/or by the charges of the same polarity on accompanying illustrative drawings, in which:

FIG. 1 illustrates an exemplary embodiment of the invention in which the moving gaseous medium is channeled in a U shaped tube;

FIG. 2 illustrates another exemplary embodiment of the invention in which the moving gaseous medium is directed along a central supply channel and diverted into two exhaunt channels;

FIG. 3 illustrates a further exemplary embodiment of the invention in which the moving gaseous medium is directed along a central supply channel and diverted into only one exhaust channel.

Referring to the drawings in detail, FIGS. 1, 2, and 3 illustrate specific uses of the method in varying apparatuses and more specifically, FIG. 1 illustrates the preferred embodiment of the invention. In FIG. 1, the system is comprised of a U-shaped tube 4 with an entrainment area 5, an annular attractor electrode 6 and a needle type corona electrode 7 near to, but downstream of, the entrance 8, and an accelerating electrode 9 at a bend zone 10 of the tube 4. Within the bend zone 10 and on the wall 11 opposite to that in which accelerating electrode 9 is placed, an aperture 12 is provided. Parallel to the bend zone 10 and at a distance from the wall 11 is placed a substrate member 13 having an upper deposition surface 14 to be coated. In contact with the under surface of member 13 is a grounded metal plate 15. The region 16 between the wall 11 of the tube 4 and the deposition surface 14 is an area of still air or zero turbulence.

In operation, particles 20 are entrained in a carrier gas stream, for example air, in the entrainment area 5, before they reach the entrance 8 of the tube 4. Such entrainment may take place, for example, by feeding the particles from a source 21 thereof through a conduit 22 and into a I chamber 23 within which the particles are sucked into the moving gaseous stream.

After passing through the entrance 8, the high velocity moving gaseous stream conveys the particles 20 through the ionizer section between the ionizing electrodes 6 and 7 where they acquire a charge. The ionizing electrodes 6 and 7 are maintained at a high potential therebetween by an external power supply 24. The particles 20 are charged to about 60% of the theoretical charge to be put on a particle of a given radius. For example, for a ten micron diameter particle, this amounts to several thousand electronic charges. The particles 20 are then transported by the moving gaseous stream to the bend Zone 10 of the tube 4, where an accelerating field has been established between the accelerating electrode 9 and the ground plate 15. The accelerating electrode 9 is maintained at a high potential relative to the plate 15 by an external power supply 25, the polarity of such potential being the same as that of the charges on the particles. The electrostatic field in the bend zone accelerates the charged particles 20 in a direction transverse to the gaseous stream flow. The charged particles 20 are ejected from the high velocity gaseous stream through the aperture 12 and move to the substrate member 13 to there deposit on the surface 14. In order to minimize the flow of the gaseous stream through the aperture 12, the flow of the gas in the tube 4- should be adjusted so that the static pressure inside the tube 4 is equal to the ambient pressure in the region of still air 16. Excess particles 20 which have not drifted transverse to the flow under the action of the accelerating field in the bend zone )10 are neutralized by a conventional collector electrode 26, passed through the exit 27 of the tube 4, and returned (via conventional means, not shown) to the particle source 21 where they can be reinjected into the moving gas stream.

By this method, the charged particles 20 are transferred from a region of high velocity flow to a region of zero turbulence air 16 where they can be deposited uniformly on a deposition surface 14. Upon emerging from the aperture 12, the forces acting on the particles 20 are: (1) the electrostatic repelling force caused primarily by the field between elements 9 and but also caused to an extent which may not be negligible by the same polarity charges on the several particles, (2) the gravitational force, and (3) the viscous drag of the air. The gravitational force, will, in general, be small in comparison to the electrostatic force, and the particles will be mainly accelerated by electrostatic repulsion until the force due to the viscous drag of the air is equal and opposite to the particle accelerating force. The particles 20 then will move at a constant velocity until they are deposited on the substrate 13.

In FIG. 1, the substrate member 13 is depicted as being a flat or elongated plate, but it is understood that it can be of any size and shape.

In order to coat the whole surface 114 of the substrate 13, the tube 4 is moved parallel to and longitudinally relative to the member 13 at a speed determined by the required deposition thickness. Such movement may be effected by mounting the tube on a guided movable carriage (not shown).

Several variables, each of which can be independently controlled, influence the rate of deposition on the substrate 13 to be coated. These are: mass rate of the particles 20 in the gaseous stream, volumetric flow and the velocity of carrier gas, degree of particle charging by the ionizing electrodes 6 and 7, the potential applied to the accelerating electrode 9 by the external power supply 25, the distance between the aperture 12 and the substrate surface 14, and the rate of travel of the tube 4 relative to that surface. The optimum combination of all these variables must be determined experimentally. The wide range of variables indicates the complete flexibility of the proposed coating system with regard to speed of coating and coating thickness.

In FIG. 2, the system is comprised of a central supply channel and two exhaust channels 31 and 32. The downstream end of the central supply chanel 30 is left open forming an aperture 33. Disposed in the aperture 33 are two ionizing electrodes, namely an annular attractor electrode 34 and a corona electrode 35 spaced from and centrally positioned within the attractor electrode 34. The ionizing electrodes 34 and 35 are maintained at a high potential therebetween by an external power supply 36. Perpendicular to the central supply channel 30 and at a distance from the aperture 33 is placed a substrate member 37 facing towards the aperture, such member 37 having a deposition surface 38 to be coated. The region 39 between the aperture 33 and the member 37 is an area of still air or zero turbulence.

lIn operation, particles 40 are entrained in a carrier gas stream, for example air, in, say, the way before described (in connection with FIG. 1) before the stream reaches the entrance 41 to the central supply channel 30. The mixture of gas and particles 40 is then flowed through the entrance 41 and down the central supply channel 30 by means of an air pump (not shown). The exhaust channels 311 and 32 are connected to a vacuum pump 42 which causes the moving gaseous stream to be diverted into those two channels. However, denser particles 40 in the gaseous stream are, due to their own inertia, ejected from the gaseous stream at the bend zone 43 and thereafter pass between the ionizing electrodes 34- and 35, through the aperture 33, and into the region of zero turbulence 39 outside of the central supply channel 30. While being conveyed between the ionizing electrodes 34 and 35, the particles 40 acquire a charge. Upon entering the region of zero turbulence 39, the charged particles 40 mutually repel each other due to space charge effects and, by such repulsion, are accelerated towards and deposited on the deposition surface 38. Excess particles 40 which are diverted (along with the gaseous stream) into the exhaust channels 31 and 32, are returned (via conventional means, not shown) to the particle feeder source 21 (FIG. 1) where they can be reinjected into the moving gaseous stream. Also, the entire particle deposition system may be moved relative to member 37 as before described in connection with FIG. 1.

Again, several variables, each of which can be independently controlled, influence the rate of deposition on the substrate member 37 These are: mass loading of the particles 40 in the gaseous stream, volumetric flow and velocity of the carrier gas, degree of particle charging by the ionizing electrodes 34 and 35, the distance between the aperture 33 and substrate member 37, and the rate of travel of the whole system parallel to and longitudinally relative to the substrate member 37.

It is to be understood, that in FIG. 2 the particles 40 can alternately be accelerated to the deposition surface 38 on the substrate member 37 by two external electrodes (as in FIG. 1) instead of relying only on space charge effects.

In FIG. 3, the system is essentially similar to that in FIG. 2 except that there is only one exhaust channel 50 diverting the gaseous stream from the supply channel 51 at the bend zone 52, and, also, the ionizing electrodes (consisting of an annular attractor electrode 53 and a corona electrode 54) are placed along the supply channel 51 upstream of the bend zone 52.

In operation, the system described in FIG. 3 is essentially similar to the operation of the system before described in FIG. 2 except that a conventional collector electrode 56 is provided to neutralize the charged particles 55 which have been diverted with the carrier gas stream into the exhaust channel 50.

The variables which influence the rate of deposition in the system shown in FIG. 3 are the same as those which influence the rate of deposition for the system shown in FIG. 2.

It is to be understood, that in FIG. 3 the particles 55 can alternately be accelerated to the deposition surface 60 on the substrate member 61 by two external electrodes (as in FIG. 1) instead of relying only on space charge effects.

We claim:

1. A method of depositing particles comprising, entraining said particles in a moving gaseous medium, exposing said medium to an ionizing electric field to effect a unipolar charging of said entrained particles, deflecting the direction of the main path of flow of said medium with said entrained particles therein at a bend zone bordered on the outer side thereof by a non-turbulent fluid region, said moving gaseous medium having along said flow path a pressure which is substantially the same as that of said non-turbulent fluid region, separating the particles from said flowing medium by at least said deflecting action so as to direct the particles out of the main path of flow of said medium and into said non-turbulent region, and driving the charged particles in said region by a repelling electric field of the same polarity as the charged particles to a deposition surface bounding said region and spaced from said bend zone.

2. A method as in claim 1 in which said repelling electric field is at least partly provided by an electrostatic interelectrode field extending in said non-turbulent region transversely of said deposition surface, said interelectrode field being polarized to drive said particles towards said surface.

3. A method as in claim 2 in which said electrostatic interelectrode field extends through said medium at said bend zone in a direction transverse to the flow direction at said bend zone of said medium, said interelectrode field being polarized to assist in driving said charged particles out of the main path of flow of said medium and into said non-turbulent region.

4. A method as in claim 1 in which said repelling electric field is at least partly provided by an electrostatic interelectrode field extending at said bend zone through said medium in a direction transverse to the flow direction of said medium at said bend zone, said interelectrode field being polarized to assist in driving said charged particles out of the main path of flow of said medium and into said non-turbulent region.

5. Apparatus comprising, tubular wall means definitive of a flow channel for a gaseous medium, said wall means being shaped to provide a bend zone for said channel, and said wall means having formed therein on the outer side of said zone a passage from said channel to a nonturbulent fluid region outwards of said wall means, said tubular wall means definitive of a flow channel for said gaseous medium having substantially the same pressure as the pressure of said non-turbulent region, means for entraining solid particles in a flow of said medium through said channel so as to impart movement to said particles by said flowing medium, ionizing electrode means disposed in relation to said moving particles to charge such particles, accelerating electrode means disposed on the inner side of said bend zone, said accelerating electrode means having the same electrical polarity as said charged particles, electroconductive plate means disposed on the outer side of said region in spaced relation from and opposite to said passage and adapted to have inserted in front thereof a substrate member disposed in said region and facing towards said passage, and electrical energy source means coupled to said accelerating electrode means and said electroconductive plate means to maintain therebetween a potential adapted to drive said charged particles transverse to said gaseous flow and through said passage into said non-turbulent region and onto said substrate member.

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